The present invention relates to an ink jet printer and, more particularly, to a printer of the type in which each of a plurality of jet drop streams is deflected to deposit drops at a plurality of print positions on a moving print receiving medium, thereby forming a print image on the medium.
A number of prior art ink jet printers have operated in a binary manner with the drops generated in each of a plurality of jet drop streams being selectively charged in dependence upon whether the drops are to be deposited upon a print receiving medium. Such a printer is shown, for instance, in U.S. Pat. No. 3,586,907, issued June 22, 1971, to Beam et al. The selectively charged jet drop streams, arranged in a row, pass downward through an electrostatic field, perpendicular to the row and to the trajectories of the jet drop streams, so as to deflect the charged drops to a catcher. Selected drops are thereby prevented from striking the print receiving medium. The uncharged drops, however, pass through the electrostatic field unaffected and are ultimately deposited on the print receiving medium. By controlling the charging of the drops in each jet drop stream, a print image is formed on the print receiving medium.
Such a printing system, in which each jet in the jet row selectively deposits drops at a single print position on the print receiving medium is limited, in the printing resolution which may be obtained, since the print positions are spaced apart by the same distances as the jets. Due to constructional limitations, the interjet spacing between jets in a row may not be reduced below a minimum distance. In order to increase printing resolution, some jet printers have used two rows of jet drop streams, with the print lines serviced by the streams in one row interlacing with the print lines serviced by the streams in the other row or rows. Such an arrangement is shown in U.S. Pat. No. 3,701,998, issued Oct. 31, 1972, to Mathis. As additional rows of jet drop streams are added to increase resolution, alignment between the various rows of jet drop streams becomes increasingly critical. Additionally, circuitry must be provided to introduce timing delays in the print control signals controlling printing by the rows of jet drop streams, since the rows are displaced along the print receiving medium in the direction of movement of the medium.
In other printers, each of the jets in a row of jet drop streams selectively deposits drops at associated ones of a plurality of adjacent print positions spaced across a print receiving medium. Such printers are shown in U.S. Pat. No. 3,877,036, issued Apr. 8, 1975, to Loeffler et al, and U.S. Pat. No. 3,641,588, issued Feb. 8, 1972, to Metz. Typically, in such a printer a lateral deflection electrode is positioned between each of the adjacent jets in the row of jet drop streams. Electrical deflection potentials are applied to the electrodes to deflect each jet in a direction parallel to the jet row to the various print positions serviced by the jet. The insertion of the electrodes between adjacent jet drop streams, however, increase the minimum interjet spacing. Since the adjacent jets must be spaced relatively far apart, each jet is required to service a substantial number of print positions across the width of the print receiving medium, limiting the effective printing speed for a given resolution.
Another approach to providing printing at a plurality of print positions with a single jet is shown in U.S. Pat. No. 3,369,252, issued Feb. 13, 1968, to Adams. The Adams patent discloses an ink jet printer in which charged drops making up an entire horizontal print line are directed initially into a trajectory parallel to a sheet of paper upon which they are to be deposited. When the row of drops is appropriately positioned above the sheet of paper, a pair of electrodes, one positioned beneath the sheet of paper and one above the jet drops, receive an electrical deflection potential which creates a field to attract the charged drops to the sheet of paper, such that the drops all strike the paper simultaneously. The deflection field is created only periodically, with a zero field being provided during times in which a row of drops is being aligned above a sheet of paper. Since a single jet is utilized to service all of the print positions across the width of the print receiving medium, the Adams printer is extremely slow.
U.S. Pat. No. 2,633,796, issued Apr. 7, 1953, to Pethick, and U.S. Pat. No. 4,126,711, issued Nov. 21, 1978, to Marlow, both disclose jet drop printers in which a charged pattern on or beneath a drop receiving surface deflects drops from a plurality of jet drop streams to desired print positions on the print receiving surface. The Pethick device utilizes a web-supporting drum, having a conductive substrate with raised portions contacting the bottom of a web. The substrate is charged and drops issuing from a mist type printer nozzle arrangement are attracted to the web in the areas above the raised portions of the substrate.
In the Marlow printer, charges are positioned on a print receiving drum in a manner similar to that utilized in xerographic printing. Drops from a plurality of nozzles are then attracted to the charged regions of the drum, thus forming the print image. The image may thereafter be transferred to a sheet of paper.
Similarly, U.S. Pat. No. 3,943,848, issued Mar. 16, 1976, to Watanabe et al discloses an ink mist printer in which electrodes positioned behind a print receiving medium are energized, as desired, to attract ink mist droplets to the medium in the region of the electrodes.
It is seen that there is a need for an ink jet printer utilizing a row of jet drop streams in which drops in each of the streams may be deflected in a direction parallel to a row to a plurality of print positions and in which deflection electrodes are not positioned between adjacent jet drop streams in the row.